Themeda arundinacea belongs to a group of giant Themeda species with stout, solid, reedlike culms and large, drooping panicles, also including T. caudata, T. intermedia, T. trichiata, and T. villosa. Most were treated in old literature at infraspecific rank under T. gigantea (Cavanilles) Hackel, which name is now applied only to a form endemic in the Philippines with awnless racemes of small, hairy spikelets. The species of this complex probably intergrade, and variation is not well understood.